<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
  .outer {
      width: 0;
      height: 0;
      border-radius: 50%;
      border: 100px transparent solid;
      border-left: 100px red solid;
  }
</style>
<body>
<div class="outer"></div>
</body>
</html>
